This invention relates to liquid crystal display devices, and particularly to active matrix liquid crystal display devices wherein a non-linear device is combined with display elements in order to improve the display characteristics.
In recent years, the field of application of twisted nematic liquid crystal display devices (hereinafter referred to as TN liquid crystal display devices) has expanded. The TN liquid crystal display devices have been utilized in great quanities in electronic equipment and small-size equipment, such as for example, wristwatches, electronic calculators, and the like. In order to increase the field of application for such display devices, it is necessary to increase the display capacitiy of the display devices. However, the liquid crystal display devices in accordance with the prior art are not fully satisfactory for this objective. The structure of conventional TN liquid crystal display devices includes a nematic liquid crystal material encapsulated between substrates so the the macroaxis of the liquid crystal molecule may be twisted at an angle of about 90.degree.. Transparent electrodes are deposited on the interior surfaces of substrates which are sandwiched between a polarizer and analyzer wherein the axis of polarization are crossed at an angle of about 90.degree.. The display is produced by the field effect of the liquid crystal material.
In order to improve the display in such devices, conventional TN liquid crystal display devices are driven in a multiplex mode. However, this multiplex driving is generally limited to only some ten lines as there is little difference between the effective voltages applied to a selected element, a non-selected element and a half-selected element. The cross talk tends to increase when the number of lines is increased. This shortcoming is caused by the non-sharpness of the voltage-contrast characteristics.
In order to increase the display capacitiy of the liquid crystal display device, an active multiplex device using a switching device or a non-linear device has been proposed. Various approaches to the non-linear devices, such as TFT or a diode consisting of amorphous silicon or polysilicon or a varistor consisting of zinc oxide have been proposed. Among the above-mentioned non-linear devices, the non-linear device utilizing a structure of metal-insulator-metal (MIM) is the most promising. The MIM is advantageous in that its manufacturing process is relatively simple as is the design of the device because of its simple structure. The above-mentioned MIM exhibits a non-linear voltage-current characteristic as shown in FIG. 1. In this case, the electric current is based on a tunnel effect, the Schottky effect or the Poole-Frenkel effect. The device utilizes an oxide layer of tantalum or tantalum nitride and has been proposed by Baraff, D. R., et al., (1980, SID International Symposium Digest of Technical Papers, Vol. XI, p. 200, April, 1980). The oxides of Al, Ta, Nb, Ti, Si, and the like or the oxides of the above-mentioned metals doped with nitrogen, an inorganic material, such as a chalcogenide glass and the like, or an organic film can be utilized as an insulator.
When the above-mentioned metallic oxides are utilized as an insulator in the MIM, the thickness of the oxide layer results in different conduction structures. It is known that the tunnel effect predominates when the insulator film is in the range of about 50-100 .ANG., that the Schottky effect and the Poole-Frenkel effect predominate when the insulator film is in the range of between about 100-1,000 .ANG.. With respect to the connection between a liquid crystal display element and a MIM, which is an object of this invention, it is desirable to utilize an insulating film in the region for exhibiting the Poole-Frenkel effect in view of the liquid crystal driving method. In this region, the above-mentioned voltage-current characeteristics is represented by the Poole-Frenkel expression as follows: ##EQU1## Where I is the electric current, V is the applied voltage, K and .beta. are constants which indicate the easiness of flow of the electric current and non-linearity, respectively.
When the liquid crystal display device including a MIM is driven by the generalized AC amplitude selective multiplexing method, which is utilized in the normal matrix driving a liquid crystal display element, the ratio of ON/OFF effective voltage which is actually applied to the liquid crystal material becomes larger than the ratio of ON/OFF effective voltage of the generalized AC amplitude selecting multiplexing method itself, because of the non-linearity of MIM. Therefore, matrix driving of many lines is possible. When a MIM is connected to the liquid crystal display element, as shown in FIG. 2, in an equivalent circuit for one picture element MIM and a liquid crystal display element are connected in series. In the MIM device, the capacitance C.sub.MIM and the non-linear resistance R.sub.MIM are in parallel, and a liquid crystal element the capacitance C.sub.Lc and the resistance R.sub.Lc are in parallel.
However, in the above-described MIM, a difference occurs in the voltage-current characteristics in response to the polarity of the onset voltage, and it is very difficult to remove this difference completely. In the case that a MIM having the above-mentioned rectification is connected to a liquid crystal display element, for example, when a MIM having the voltage-current characteristics as shown in FIG. 3 is coupled to a liquid crystal display element and is driven by the generalized AC amplitude selective multiplexing method in 1/5 bias, the waveform of the voltage actually applied to the liquid crystal has a polarity difference because of the rectification of the voltage-current characteristics of the MIM (A and B portions shown in FIG. 5). In this case the same waveform when applied to the liquid crystal material is not an asymmetrical alternating waveform, but an alternating waveform biased by direct current (DC). When the liquid crystal material is driven by direct current, the electrochemical reaction within the liquid crystal material itself and of the impurities therein increase. Such occurrences are undesirable as they adversely effect duration of the usefulness of the liquid crystal display device.
The equivalent circuit of a MIM element and a liquid crystal picture element in a twisted nematic liquid crystal panel show the MIM as a non-linear resistor R.sub.MIM in parallel with capacitor C.sub.MIM and the liquid crystal picture element as a resistor R.sub.Lc in parallel with a capacitor C.sub.Lc. When the driving voltage is applied to both ends of the MIM and the liquid crystal picture element, the effective voltage applied to the liquid crystal picture element actually depends on the combination of the capacitor C.sub.MIM of the MIM and the capacitor C.sub.Lc of the liquid crystal picture element. According to this calculation, as the value of the capacitance C.sub.MIM of the MIM is less than that of the capacitor C.sub.Lc of the liquid crystal picture element, the permitted design limit of the MIM becomes broader. The value of the ratio of the capacitance C.sub.Lc /C.sub.MIM is between about 5 to 20. The larger this value is, the better.
According to the designs of Baraff, the liquid crystal picture element is 1.25 mm pitch and the size of the MIM is 12 .mu.m.times.12 .mu.m. A size between 0.3 and 0.5 mm pitch for an active matrix liquid crystal panel is generally used as a practical matter in many devices. Therefore, the MIM would have a dimension from 3 to 5 .mu.m square within the same design limitations.
In order to obtain uniform optical characteristics in all parts of the liquid crystal panel, the characteristics of each MIM should be the same on the substrate. However, the dimension of 3 to 5 .mu.m is difficult to obtain utilizing conventional photolithography mask aligners, but may be obtained by a fine patterned process or VLSI. Accordingly, it will be necessary to utilize high precision mask aligners in order to provide each MIM with a uniform area. As it is an object of the invention to increase the overall size of the twisted nematic liquid crystal panel including the MIM elements, this high precision mask aligning results in greater manufacturing costs.
Accordingly, it is desirable to provide an active matrix liquid crystal display device including MIM elements and processes for their preparation which avoid the disadvantages noted above. Two MIM elements connected in parallel with each other to offset rectification are connected to each liquid crystal display element in series provides symmetrical voltage-current characteristics and avoids the need of highly accurate photolithographical patterning by increasing the area of the MIM element.